In recent years, in a calculator system widely used as information equipment such as a mobile phone, a slate device or a tablet terminal and a large-scale system such as a supercomputer, a reduction of system power consumption has become an important issue. As a method of reducing power consumption, for example, a power gating technique of supplying power to only portions (units) which requires power is known.
For example, in an idle state during which the processor of a calculator system executes nothing, it is possible to reduce power consumption by stopping the supply of power to a cache memory in which part of plural pieces of data used for processing of the processor is stored. Here, since the cache memory is generally configured as volatile memory such as RAM, when the supply of power to the cache memory is stopped, all of the data stored in the cache memory is lost. Thus, when stopping the supply of power to the cache memory, it is necessary to perform a writeback process which involves writing overwritten data (data of which the content is not identical to that stored in a main storage device) among the data stored in the cache memory back to the main storage device.
However, the power consumption amount associated with the writeback process may be sometimes greater than the power consumption amount when the supply of power to the cache memory is not stopped. In this case, performing the writeback process to stop the supply of power to the cache memory does not lead to a reduction of the power consumption amount. That is, in a configuration in which the writeback process is always performed in the idle state to stop the supply of power to the cache memory, there is a problem in that it is difficult to reduce the power consumption amount efficiently.